On Mar 19, 6:20 pm, Erik Wikstr?m <Erik-wikst...@telia.com> wrote:
On 2008-03-19 11:49, James Kanze wrote:
[...]
There are a number of useful values in either <cfloat> or
<limits>. Those in <cfloat> are macros which evaluate to
constant expressions, which when integral can be used in the
preprocessor, e.g. for conditional compilation. (The ones that
would interest you here are xxx_RADIX, xxx_MANT_DIG, xxx_MIN_EXP
and xxx_MAX_EXP, where xxx is one of FLT, DBL OR LDBL.)
Or one can mandate IEEE 754 (also known as IEC 559) by using
the is_iec559() function of the numeric_limits class. Combine
this with sizeof() and all you have to worry about is
endianess issues (which you might be able to solve using
ntohl() in POSIX).
But that's runtime, and not compile time.
In C99 (and in C++0x?), you can test whether __STDC_IEC_559__ is
defined. In practice, however, of the five compilers I have
ready access too (VC++ and g++ under Windows, g++ under Linux,
and g++ and Sun CC under Solaris), only g++ under Linux tests
true, although all of the machines in question are IEEE. The
preprocessor symbols I mentionned have been around since C90,
however, and are pretty much universally supported. (The also
permit testing for other formats, in case you only want to
support IBM format. Or for something like:
#if FLT_RADIX == 2 && FLT_MANT_DIG == 24 && ...
// IEEE implementation
You'd need to test a lot more than just those two macros, of course.
#elif FLT_RADIX == 16 && FLT_MANT_DIG == 6 && ...
// IBM implementation
#else
#error floating point format not yet supported
#endif
I'd do this very differently, of course. Something more like:
#include GB_dependentInclude(floatingpoint,gb/whatever.cc)
with the various implementations in separate source files, in
different directories. Conditional compilation is an anathema
for maintenance, and should generally be avoided.